Here at Cape Tech we have gone in all directions to help reduce our carbon footprint, and save money. One of the big contributors to our school’s energy savings is the Tri-gen, a generator which is an eco-friendly alternative to more conventional energy sources. The tri-gen was implemented in the school in late September 2007, and is located next to the REC building. It generates about 250 kilo-watts (kW) of energy, or about 25% - 50% of the school’s energy needs. It runs solely on natural gas, which is competitive in price to gasoline and has a less negative effect on the environment. In the tri-gen system, energy in the form of heat isn’t lost like in conventional systems.
“Conventional thermoelectric stations convert only 1/3 of the fuel energy in electricity. The rest is losses in the form of heat.” (“Us Green Energy, inc”) The cogeneration (tri-gen) system provides us with thermal energy for the school, and 250 kW of electrical energy. Cogeneration means that at the same time that electrical energy is being produced; thermal energy is also being produced to heat or cool the building. For our school, the Nimbus 250L system was selected. This system is especially good for our school because it acts as a hurricane shelter for the community. The 250 kW that is generated by the tri-gen is able to sustain most of the energy load created by the school at normal operation.
“The rising prices (of oil) really made us look hard at what we had been doing and what we could be doing differently,” says Bob Sanborn, business administrator at the Cape Cod Regional Technical High School. The low risk lease program that our school acquired on the tri-gen enables the school to set a normal budget for oil as if our school didn’t have the tri-gen. The money that is not used by the utility company to purchase oil is used to pay off the tri-gen month by month.
Why does the tri-gen only provide 250 kW of energy to the school? The tri-gen has to be sized up according to the thermal energy the building uses, not its electrical needs. The reason being is for a tri-gen to provide maximum output, it needs to be working all of the time. For example, when the tri-gen is providing more heat than the school needs, the tri-gen becomes less efficient because it can’t store that thermal energy and therefore loses that energy. In order to compensate for that, Aircogen, the designers of the Tri-gen, have implemented a dump-mode when the tri-gen is providing too much thermal energy. For example, when the school is hot enough or cool enough, the tri-gen will switch over to 30% efficiency, or dump-mode, instead of 85% efficiency of normal use. Although they have designed the dump-mode, they don’t intend to ever use it due to the correct sizing of the tri-gen. Aircogen has estimated that the school will use 100% of its output all the time.